1. Field of the Invention
The present invention concerns a method and device for measuring the spectral phase or the combined spectral and spatial phases of ultra short light pulses.
2. Description of the Prior Art
It primarily concerns spectral phase measurements, i.e. variations in the phase according to the frequency in the spectrum of these pulses. Secondarily, it relates to the concomitant measurement of the spatial phase, i.e. phase variations according to the position in a plane perpendicular to the direction of propagation. Indeed, simultaneous measurements of spectral and spatial phase, called spatial-temporal measurements, are important to characterize the sources of ultra short pulses and related devices such as compressors and temporal extenders.
In general, one knows that the measurement of the amplitude and phase of ultra short light pulses with durations between several femtoseconds and several picoseconds, presents many difficulties.
Various measurement methods of the prior art are described in the following documents:    I. A. Walmsley and R. Trebino: “Measuring fast pulses with slow detectors”, Optics and Photonics News, March 1996, vol. 7, No. 3, p. 23 hereinafter designated (WT),    C. Dorrer and M. Joffre: “Characterization of the spectral phase of ultrashort light pulses”, C.R. Acad. Sc. Paris, t.2, series IV, p. 1415-1426, 2001 hereinafter designated (DJ).
When one has a reference pulse of known phase, a simple method described in the (DJ) document to measure the spectral phase consists of superimposing this reference pulse on the pulse to be measured offset temporally. The spectrum of a pulse of this type has oscillations with amplitude whereof one can deduce the phase difference between the pulse to be measured and the reference pulse for each wavelength. This method will be called simple spectral interferometry (SSI). It uses only linear optical interactions. It can be applied jointly to the spectral phase and the spatial phase.
In general, however, one does not have this type of reference pulse and the prior art comprises various methods called “self-referenced”. It is necessary for all of these methods to use at least one nonlinear response optical element. This is recalled in particular in the (DJ) document.
Among the self-referenced methods, one can cite the FROG (Frequency Resolved Optical Gating) method and the SPIDER (Spectral Phase Interferometry for Direct Electric Field Reconstruction) method. These two methods are described in the following documents, respectively:    R. Trebino and D. J. Kane: “Using phase retrieval to measure the intensity and phase of ultrashort pulses: Frequency Resolved Optical Gating”, J. Opt. Soc. Am. A11, p. 2429-243′7, 1993, with regard to the FROG method,    C. Iaconis and I. A. Walmsley: “Spectral Phase Interferometry for Direct Electric field Reconstruction of ultrashort optical pulses”, Opt. Lett, 23, p. 729-794, 1998, with regard to the SPIDER method.
In all cases, several replicas of the initial pulse, spectrally modified or not, are mixed non-linearly in order to obtain the useful signal. The methods differ depending on whether they need a single measurement (one-shot measurement) or several measurements corresponding to successive light pulses. In the latter case, it is necessary for these successive pulses to be essentially identical. The methods also differ depend on whether the phase can be derived from measurement through a direct algorithm, as is the case for (SSI), or whether they use a successive adjustment procedure aiming to minimize the difference between a calculation of the expected measurement for a test spectral phase and the measurement itself. The FROG method, for example, uses a technique of successive adjustments, while the SPIDER method allows the use of a direct algorithm. The direct algorithm is in general considered to be preferable given the possible uncertainties on the convergence of the successive adjustments.
Moreover, the methods differ as to their ability to perform a spatial-temporal measurement as discussed above. The FROG method does not allow this combined measurement without ambiguities between temporal and spatial. A spatial-temporal measurement configuration, from a one-shot SPIDER method, was done at the cost of a greatly increased complexity of optical assembly and to the detriment of the instrument's sensitivity. It is described in the following document:    C. Dorrer, E. M. Kosik, I. A. Walmsley: “Spatio-temporal characterization of the electric filed of ultrashort optical pulses using two-dimensional shearing interferometry”, App. Phys. B, 74, p. 209-21′7, 2002.
It must be noted that in French patent 02 05872 “Method and device for measuring the phase and amplitude of ultra short light pulses”, it was proposed to realize various self-referenced methods from the prior art in a single instrument comprising a pulse shaper, a non-linear element and a detector element. The pulse shaper is a device making it possible to apply a programmable linear filter to a pulse, i.e. to modify the phase and spectral amplitude of this pulse in a controlled manner. Among the devices of the prior art, one distinguishes those based on a zero-dispersion “4f” line configuration, as described in the publication of D. E. Leaird, and A. M. Weiner, “Femtosecond direct space-to-time pulse shaping,” IEEE Journal of Quantum Electronics, vol. 37, pp. 494-504, (2001) and those founded on an acoustic-optical programmable dispersive filter (AODPF) as described in French patent 96 08510, “Device for controlling light pulses via a programmable acoustic-optical device”.